Effect of spatial confinement on the glass-transition temperature of patterned polymer nanostructures

Poly(methyl methacrylate) (PMMA) nanostructures embedded with a fluorescence tag are fabricated using electron beam lithography on oxidized silicon substrates. The glass transition temperatures (Tgs) of these one-dimensional (1-D) nanostructures (parallel lines) are measured by monitoring their temperature-dependent fluorescence intensities, revealing substantial differences between the Tgs of the nanostructures and the thin films from which they were fabricated. For example, the Tg of 50-nm-wide PMMA nanolines on silica is approximately 15 K lower than that of a PMMA film on silica of the same 18 nm thickness. Attractive PMMA-silica interfacial interactions increase the Tg, while free surfaces decrease the Tg of PMMA in ultrathin films relative to bulk PMMA. Thus, the significant differences between the Tgs of the 1-D and two-dimensional (2-D) forms of PMMA on silica are the result of a substantial increase in the ratio of free-surface area to interfacial area in the PMMA nanolines relative to ultrathin films.